Variable lenses, e.g., multiple focus lenses and zoom lenses have traditionally employed one or more non-deformable (i.e., rigid such as glass or polycarbonate) lens elements which are moved along an imaging axis by forces often supplied by a motor.
In recent years, motorless electro-responsive lens elements have attracted increased attention of researchers and designers of optical systems. One type of motorless electro-responsive lens element is the “fluid lens” lens element which generally includes a rigid or elastomeric membrane filled with one or more fluids having indices of refraction greater than 1. Fluid lens element technology has attracted the attention of many designers of optical systems who generally see traditional solid lens elements and motor equipped systems as bulky and energy hungry. With the proposals for fluid lens elements there have been proposed various methods for varying an optical property of a fluid lens element for integration into an optical system. Where fluid lens elements have been proposed, the proposed alternatives for varying optical properties of such lens elements can be categorized into three broad categories: electro wetting, fluid injection, and mechanically actuation.
According to a process of electro wetting, a fluid lens element is provided having at least two immiscible fluids and a voltage is applied to the fluid lens element. A surface tension of the fluid lens element changes as a result of the voltage being applied, bringing about a change in the curvature of an interface between the at least two fluids.
According to a process of fluid injection, a pump is provided adjacent a fluid lens element which pumps in and draws out fluid from the lens element. As fluid is pumped in and drawn out of the lens element, optical properties of the lens element change.
According to a process of mechanical actuation, a lens element is provided having a deformable membrane being secured within a housing, and a focus fluid confined to a cavity defined by the housing, the membrane, and an end plate. An actuator imparts a force to the membrane, which changes the geometry of the cavity holding the focus fluid.
Problems have been noted with all three methods for varying an optical property of a fluid lens element. Regarding electro wetting, one problem that has been noted is that the electrical current repeatedly flowing through the lens element tends to alter the characteristics of the lens element over time, rendering any system in which the lens element is employed unreliable and unpredictable. Another problem noted with proposals involving electro wetting is that electro wetting normally involves providing two types of fluids. As the reference index difference between the fluids is small, the power of the lens element is reduced.
Regarding the fluid injection methods, the pumps for providing such fluid injection are necessarily complex and intricate making a reasonably costly system and acceptable miniaturization difficult to achieve.
Regarding the mechanical actuation methods, manufacture of the lens element assembly has proven problematic due to the difficulty in handling the focus fluid. Furthermore, the assembly cannot sustain high temperature operation (e.g., above 45 degrees Celsius) due to the weak structure of the deformable membrane with focus fluid inside the cavity. Further, the assembly is susceptible moisture and environmental effects, thereby limiting the lifespan. For example, certain material choices for the deformable membrane attract dust. Finally, and most critically, the fluid lens is susceptible to leakage over the lifetime of usage.
Because of the problems noted with the electro wetting, fluid injection, and mechanical actuation methods for varying an optical property of a deformable lens element, designers of commercially deployed optical systems continue to rely almost exclusively on traditional motor-actuated rigid lens elements in the design of optical systems. Yet, the miniaturization and energy conservation achievable with motor-actuated rigid element equipped optical systems continues to be limited.